Most people like to evaluate themselves using various tests, formulas and indexes — it is always interesting to learn something new about yourself. It is not surprising that there are many indices in the world for calculating normal weight. But does at least one of them give real data on what should be the ideal weight of a given person?
MedAboutMe breaks down the four most popular ideal weight calculation methods and finds out what are their drawbacks and should they be trusted?
Body mass index
Body mass index (BMI, BMI) is the most popular and best known formula for estimating a person’s body weight. The formula includes an indicator of weight and height: weight in kilograms should be divided by the square of height in meters (kg / m2). Accordingly, we get:
- BMI less than 18.5 kg / m2 — a person is underweight.
- BMI from 18.5 to 24.9 kg / m2 — ideal weight.
- BMI from 25 to 29.9 kg / m2 — overweight.
- BMI over 30 kg/m2 — obesity.
The biggest plus of BMI is its simplicity. Anyone can measure their height and weight with the help of the simplest tools that are always at hand.
The biggest disadvantage of BMI is that it does not take into account the full variety of variations in the human body.
- BMI does not reflect the changes that occur with age, when muscles atrophy, the amount of fat increases, and a person’s height becomes smaller due to degenerative processes in the spine. That is, BMI for older people will show an underestimated result.
- BMI does not take into account the body structure of athletes, in which the percentage of lighter fat is low, and the proportion of heavier muscles is high. BMI for athletes will show an overestimated result.
- Not all BMI calculators offer to indicate the gender of a person. But it is known that a woman with equal height with a man will have more fat, but less muscle, and together the ratio of these tissues will significantly affect the final BMI.
In a 2001 study by a group of scientists from Italy and Holland, it was shown that the sensitivity and specificity of BMI as a method of determining the level of body fat is quite low.
Thus, according to BMI calculations, obesity was detected in 25% of men participating in the study and in 35% of women. However, a survey using the densitometry method showed that 8% of men and 7% of women from the total sample who fell into the obesity group according to BMI did not actually have it, indicating a low specificity of the method.
When assessing the sensitivity of BMI, the results were even worse: false-negative BMI results were found for 41% of men and 32% of women.
Waist-to-hip ratio (WHR), or waist/hip index (WHI) is a fairly well-known method for assessing the degree of abdominal obesity, that is, obesity in which fat is actively deposited in the abdomen.
In order to calculate it, you should measure the waist in its narrowest part (if there is none, 2 cm above the navel), and divide the resulting value by the volume of the hips in their widest part.
The higher the degree of abdominal obesity and the higher the TTB score, the higher the risk of developing diabetes and cardiovascular disease.
Since the figures in men and women have pronounced gender characteristics, FTI should be considered separately for men and separately for women.
For women, the following data can be used:
- FTI less than 0.80 — normal weight.
- FTI from 0.80 to 0.84 — overweight.
- FTI more than 0.85 — obesity.
For men, the results will be as follows:
- FTI less than 0.90 — normal weight.
- FTI from 0.90 to 0.99 — overweight.
- FTI more than 1.00 — obesity.
The most significant plus of ITB is the assessment of cardiac risk, which is highly correlated with gradation by weight. In accordance with the above data, with normal weight, the risk of developing heart and vascular diseases is low, with excess weight — moderate, and with obesity — high. TPI is also used to assess the risk of type 2 diabetes.
The most significant disadvantage of ITB is that this indicator does not take into account the features of the human figure. It also does not allow you to estimate the percentage of fat in the human body and compare it with the proportion of muscle mass.
In 2016, scientists from Malaysia published an article in the International Journal of Preventive Medicine proving that the measurement of waist circumference (WC, WC) is a more accurate indicator of abdominal obesity compared to FTB. Scientists explain this by the fact that ITB can be underestimated in people with a high hip circumference and overestimated if the hip circumference, on the contrary, is small compared to the average data (this is especially true for women).
The ratio of waist circumference to a person’s height, or waist / height index (WTI, WtHR) is another attempt in a simple way to identify people with parameters dangerous to health.
According to research in this area, ITR is more effective in predicting cardiac risk, the likelihood of diabetes, and all-cause mortality than BMI or even ITB.
The criteria is very simple:
- if the ratio of waist circumference to height is 0.5 or less, then the person has a normal weight;
- if the MRI is greater than 0.5, this indicates the presence of risks to his health.
Advantages of measuring MRI: simple and convenient for approximate risk assessments of developing heart and vascular diseases, as well as diabetes mellitus.
Cons of ITR: all the same as for other indices. The features of the figure and physical activity of a person are not taken into account.
In order to calculate the weight norm according to Brock’s formula, it is enough to subtract a certain figure from your height. And which one depends on growth:
- If you are 165 cm or less: Subtract 100 from your height.
- With a height of 166–175 cm: subtract 105 from the height.
- With a height of 176 cm or more: subtract 110 from the height.
But that’s not all! Consider what type of physique a person has. And as you know, there are three of them (main): asthenics (thin and tall), normasthenics (harmonious), hypersthenics (massive and low). Then, when using Broca’s formula, after subtracting the correct figure, it follows:
- Asthenics deduct another 10%.
- Normostenics — do not change anything.
- Hypersthenics — add 10% on top.
But this still does not give the final value. Because people come in different ages, and, as mentioned earlier, their height and, to some extent, physique depend on age. Therefore, plus everything follows:
- For young people aged 20–30 years — further reduce the calculated figure by 10%.
- People aged 40–45 years — do not change anything.
- Elderly people over 50 years old — add 5–7% on top.
The idea of Brock’s formula is good: take into account age, body type, features of the figure with different heights — and this is a plus of this formula. It is already difficult to call it simple, given the numerous edits that occur in the process.
There is a slightly simplified modern version of Brock’s formula using a factor of 1.15:
- To calculate the weight of a woman, it is necessary: subtract 110 from height and multiply the resulting number by 1.15.
- To calculate the weight of a man, you need to subtract 100 from height and multiply the resulting number by 1.15.
The minus of Brock’s formula is just in too many nuances, and also in the fact that in this case it is impossible to take into account the ratio of fat and muscles. The same athletes, pregnant ladies and girls under 18, for example, will again be outside the norm.
- There is no ideal formula for calculating the “correct” weight with a minimum of data and measuring instruments. People are too different.
- All these methods give very approximate results, especially at the borders of the ranges. Especially for people who are somehow different from the average person — height, health, body condition, etc.
- Nevertheless, these indices are quite suitable for understanding: the risks of heart and vascular diseases, diabetes and other dangerous diseases are already too high, and something needs to be done with extra pounds.
- A more accurate result can be obtained by measuring the proportion of fat and muscle in the body.
Comparison of selected body composition parameters in women using DXA and anthropometric method. / Grzegorczyk J, Woloszyn N, Perenc L. // J Res Med Science - 2019 Aug - 28;24:70
Abdominal Obesity Indicators: Waist Circumference or Waist-to-hip Ratio in Malaysian Adults Population. / Ahmad N, et al. // Int J Prev Med. - 2016 - 7:82
BMI-related errors in the measurement of obesity. / Rothman KJ. // Int J Obes (Lond). - 2008 Aug - 32 Suppl 3:S56‑9.
The validity of predicted body fat percentage from body mass index and from impedance in samples of five European populations. / Deurenberg, P., Andreoli, A., Borg, P. et al. // Eur J Clin Nutr - 2001 - 55